Atomic Absorption Spectroscopy.ppt

Atomic Absorption Spectroscopy THE ATOM AND ATOMIC SPECTROSCOPY The science of atomic spectroscopy has yielded three techniques for analytical use: atomic emission, atomic absorption, and atomic fluorescence. To understand the relationship of these techniques to each other, it is necessary to have...

Atomic Absorption Spectroscopy THE ATOM AND ATOMIC SPECTROSCOPY The science of atomic spectroscopy has yielded three techniques for analytical use: atomic emission, atomic absorption, and atomic fluorescence. To understand the relationship of these techniques to each other, it is necessary to have an understanding of the atom itself and of the atomic process involved in each technique. The atom is made up of a nucleus surrounded by electrons. Every element has a specific number of electrons associated with the atomic nucleus in an orbital structure unique to each element. The electrons occupy orbital positions in an orderly and predictable way. The lowest energy, most stable electronic configuration of an atom, known as the ‘‘ground state’’, is the typical orbital configuration for an atom. If the energy of the correct magnitude is applied to an atom, the energy will be absorbed by the atom, and an outer electron will be promoted to a less stable configuration or ‘‘excited state’’. As this state is unstable, the atom will immediately and spontaneously return to its ground state configuration. The electron will return to its initial, stable orbital position, and radiant energy equivalent to the amount of energy initially absorbed in the excitation process will be emitted. The process is illustrated in Figure 1-1. Note that in Step 1 of the process, the excitation is forced by supplying energy. The decay process in Step 2, involving the emission of light, occurs spontaneously. The wavelength of the emitted radiant energy is directly related to the electronic transition which has occurred. Since every element has a unique electronic structure, the wavelength of light emitted is a unique property of each individual element. As the orbital configuration of a large atom may be complex, many electronic transitions can occur, each resulting in the emission of a characteristic wavelength of light, as illustrated in Figure 1-2. What is AAS ? The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108) The application of atomic absorption spectra to chemical analysis The first commercial atomic absorption spectrometer was introduced in 1959 Alan Walsh 1916-1998 4  Introduction Atomic Absorption Spectrophotometry, which are standard instruments for the determination of metal elements, are widely applied of samples, such as agriculture chemical, clinical and biochemistry, minerals, food and drugs, environmental and other.  Principle of Atomic Absorption Spectrophotometer Principle of the Atomic Absorption Method Atomized elements each absorb energy of a wavelength that is peculiar to that element. The atomic absorption method uses as its light source a hollow cathode lamp which emits light of a wavelength that is peculiar to each element. Light absorption Elements within a solution are heated in process of atoms a flame or electrically (2000K to 3000K) and subsequently determined using the fact that the degree of absorption will vary with its concentration.  Instrumentation Line Atomizatio Monochromat source n or Detector Nebulize Read-out r Schematic diagram of a flame spectrophotomer 7 LIGHT SOURCES An atom absorbs light at discrete wavelengths. To measure this narrow light absorption with maximum sensitivity, it is necessary to use a line source that emits the specific wavelengths that can be absorbed by the atom. Narrow line sources provide high sensitivity and make atomic absorption a very specific analytical technique with few spectral interferences. The two most common line sources used in atomic absorption are the ‘‘hollow cathode lamp’’ and the ‘‘electrodeless discharge lamp.’’ The Hollow Cathode Lamp The hollow cathode lamp is an excellent, bright line source for most of the elements determinable by atomic absorption. Figure shows how a hollow cathode lamp is constructed. The cathode of the lamp frequently is a hollowed-out cylinder of the metal whose spectrum is to be produced. The anode and cathode are sealed in a glass cylinder normally filled with either neon or argon at low pressure. At the end of the glass cylinder is a window transparent to the emitted radiation.  Hollow cathode lamp (HCL) Cathode--- in the form of a cylinder, made of the element being studied in the flame Anode---tungsten 9 A hollow cathode lamp for Aluminum (Al) 10 The emission process is illustrated in Figure 2-4. When an electrical potential is applied between the anode and cathode, some of the fill gas atoms are ionized. The positively charged fill gas ions accelerate through the electrical field to collide with the negatively charged cathode and dislodge individual metal atoms in a process called ‘‘sputtering’’. Sputtered metal atoms are then excited to an emission state through a kinetic energy transfer by impact with fill gas ions. Hollow cathode lamps have a finite lifetime. Adsorption of fill gas atoms onto the inner surfaces of the lamp is the primary cause of lamp failure. As fill gas pressure decreases, the efficiency of sputtering and the excitation of sputtered metal atoms also decreases, reducing the intensity of the lamp emission. To prolong hollow cathode lamp life, some manufacturers produce lamps with larger internal volumes so that a greater supply of fill gas at optimum pressure is available. The sputtering process may remove some of the metal atoms from the vicinity of the cathode to be deposited elsewhere. Lamps for volatile metals such as arsenic, selenium, and cadmium are more prone to rapid vaporization of the cathode during use. While the loss of metal from the cathode at normal operating currents (typically 5-25 milliamperes) usually does not affect lamp performance, fill gas atoms can be entrapped during the metal deposition process, which affects lamp life. Lamps which are operated at highly elevated currents may suffer reduced lamp life due to depletion of the analyte element from the cathode. Some cathode materials can slowly evolve hydrogen when heated. As the concentration of hydrogen in the fill gas increases, a background continuum emission contaminates the purity of the line spectrum of the element, resulting in a reduction of atomic absorption sensitivity and poor calibration linearity. To eliminate such problems, most modern hollow cathode lamps have a tantalum ‘‘getter’’ on the anode which irreversibly adsorbs evolved hydrogen as the lamp is operated. The cathode of the hollow cathode lamp is usually constructed from a highly pure metal resulting in a very pure emission spectrum of the cathode material. It is sometimes possible, however, to construct a cathode or cathode insert from several metals. The resulting ‘‘multi- element’’ lamp may provide superior performance for a single element or, with some combinations, may be used as a source for all of the elements contained in the cathode alloy. However, not all metals may be used in combination with others because of metallurgical or spectral limitations. Special consideration should be given before using a multi-element lamp as analytical complications may result. Often, the emission intensity for an element in a multi-element lamp is not as great as that observed for the element in a single-element lamp. This loss of intensity could be a disadvantage in applications where high precision or low detection limits are required. The Electrodeless Discharge Lamp The hollow cathode lamp is an entirely satisfactory source for atomic absorption for most elements. In a few cases, however, the quality of the analysis is impaired by the limitations of the hollow cathode lamp. The primary cases involve the more volatile elements where low intensity and short lamp life are a problem. The atomic absorption determination of these elements can often be dramatically improved with the use of brighter, more stable sources such as the ‘‘electrodeless discharge lamp’’. Figure 2-6 shows the design of the Perkin-Elmer System electrodeless discharge lamp (EDL). A small amount of the metal or salt of the element for which the source is to be used is sealed inside a quartz bulb. This bulb is placed inside a small, self-contained RF generator or ‘‘driver’’. When power is applied to the driver, an RF field is created. The coupled energy will vaporize and excite the atoms inside the bulb, causing them to emit their characteristic spectrum. An accessory power supply is required to operate an EDL. Electrodeless discharge lamps are typically much more intense and, in some cases, more sensitive than comparable hollow cathode lamps. They, therefore, offer the analytical advantages of better precision and lower detection limits where an analysis is intensity- limited. In addition to providing superior performance, the useful lifetime of an EDL is typically much greater than that of a hollow cathode lamp for the same element. It should be noted, however, that the optical image for the EDL is considerably larger than that in a hollow cathode lamp. As a result, the performance benefits of the EDL can only be observed in instruments with optical systems designed to be compatible with the larger image. Electrodeless discharge lamps are available for a wide variety of elements, including antimony, arsenic, bismuth, cadmium, cesium, germanium, lead, mercury, phosphorus, potassium, rubidium, selenium, tellurium, thallium, tin and zinc. Atomization method Atomic absorption spectrometry measures absorption of free atom. “Free atom” means an atom not combined with other atoms. Elements in the sample to be analyzed are not in the free state, and are combined with other elements invariably to make a so-called molecule. Atomization method The combination must be cut off by some means to free the atoms. This is called “atomization” 2 types: - Flame method - Flameless method Electrothermal atomization Hydride atomization Cold-Vapor atomization  Flame atomization Processes occurring during In a atomization flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with gaseous fuel, and carried into a flame where atomization occurs. The following processes then occur in the flame. Desolvation: Solvent evaporates to produce a finely divided solid molecular aerosol. The aerosol is then volatilized to form gaseous molecules. Dissociation (leads to an atomic gas) 17 Ionization (to give Types of Flames: Common fuels and oxidants used in flame spectroscopy 18 Fuel and oxidant Auxiliary flame oxidant Air- propane Fuel Air- hydrogen  Air – acetylene  Nitrous oxide – acetylene 19 Flame Structure: Significant regions of a flame include: 1. primary combustion zone 2. interzonal region 3. secondary combustion zone The appearance and relative size of these regions vary considerably with the fuel-to- oxidant ratio as well as with the type of fuel and oxidant. 1.Primary combustion zone: is recognizable by its blue luminescence arising from the band emission of C2, CH and other radicals, in a hydrocarbon flame. Thermal equilibrium is usually not achieved in this region and is rarely used for flame spectroscopy. 2.Interzonal region: This area is relatively narrow in stoichiometric hydrocarbon flames and may reach several centimeters in height in fuel-rich acetylene-oxygen or acetylene- nitrous oxide sources. Since it is often rich in free atoms, it is the most widely used part of the flame for spectroscopy. 3.Secondary combustion zone: In the Total Consumption Burner In a total-consumption burner, the fuel and oxidant (support) gases are mixed and combust at the tip of the burner. The fuel (usually acetylene), oxidant (usually air) and sample all meet at the base of the flame. The ‘Venturi Effect’ uses the support gas to draw the sample into the flame. The gas creates a partial vacuum above the capillary barrel, causing the sample to be forced up the capillary. It is broken into a fine spray at the tip where the gases are turbulently mixed and burned. This is the usual process of ‘nebulisation’. The burner is called total consumption because the entire aspirated sample enters the flame or in other words, the sample solution is directly aspirated into the flame. All Desolvation, atomization, and excitation occur in the flame. However, the total consumption burner can be used to more easily aspirate viscous and ‘high solids’ samples, such as undiluted serum and urine. Also, this burner can be used for most types of flames, both low- and high-burning velocity Pre-Mix Burner System The spectrometer’s sample cell, or atomizer, must produce the ground state atoms necessary for atomic absorption to occur. This involves the application of thermal energy to break the bonds that hold atoms together as molecules. While there are several alternatives, the most routine and widely applied sample atomizer is the flame. Figure shows an exploded view of an atomic absorption burner system. In this ‘‘premix’’ design, the sample solution is aspirated through a nebuliser and sprayed as a fine aerosol into the mixing chamber. Here, the sample aerosol is mixed with fuel and oxidant gases and carried to the burner head, where combustion and sample atomisation occur. Schematic of Premix Burner Flame atomization Nebulizer - burner A typical premix burner 23 The sample solution is aspirated through a capillary by the ‘Venturi effect’ using the support gas for the aspiration. Large droplets of the sample condense and drain out of the chamber. The remaining fine droplets mix with the gases and enter the flame. As much as 90% of the droplets condense out, leaving only 10% to enter the flame. The 90% of the sample that does not reach the flame will travel back through the mixing chamber and out as a waste drain. The premix burners are generally limited to relatively low-burning velocity flames. The most outstanding disadvantage of the premix burner is that only low- burning-velocity flames can be used. A burning velocity which is higher than the rate of flow gases leaving the burner will cause the flame to travel down into the burner resulting in an explosion commonly known as flashback. Because of this limitation ox it is Nebuliser - burner To convert the test solution to gaseous atoms Nebuliser --- to produce a mist or aerosol of the test solution Vaporising chamber --- Fine mist is mixed with the fuel gas and the carrier gas Larger droplets of liquid fall out from the gas stream and discharged to waste Burner head --- The flame path is about 10 –12 cm 25 The difference between total-consumption burner and premix chamber burner. a) Nebulisation process In a total-consumption burner, the fuel (usually acetylene), oxidant (usually air) and sample all meet at the base of the flame. The sample is drawn up into the flame by the ‘Venturi Effect’, by the support gas. The gas creates a partial vacuum above the capillary barrel, causing the sample to be forced up the capillary. It is broken into a fine spray at the tip where the gases are turbulently mixed and burned. This is the usual process of ‘nebulisation’. While in premix burners, the fuel and support gases are mixed in a chamber before they enters the burner head (through a slot) where they combust. The sample solution is again aspirated through a capillary by the b) Size of sample droplet that enters the flame (atomization efficiency) and absorption pathlength The total consumption burner obviously uses the entire aspirated sample, but it has a shorter path length and many larger droplets are not vaporized in the sample. The path length is extremely short since combustion occurs only at a point above the capillary tube. Although in the total-consumption burners, the entire sample is aspirated, the vaporization and atomization is poor. Although a large portion of the aspirated sample is lost in the premix burner, the ‘atomization efficiency’ (efficiency of producing atomic vapour) of that portion of the sample that enters the flame is more significant, because the droplets are finer. Also, the path length is longer. The sample which does reach c) Interference to flame In total consumption burner, the larger droplets may vaporize partially, leaving solid particles in the light path. This may result in light scattering, which is registered as an absorbance. The absorbance by the sample, that is, the atomic vapour population, is generally more dependent on the gas flow rates and the height of observation in the flame than with the premix burners. The sample’s viscosity will more significantly affect the atomization efficiency (production of atomic vapour) in the total consumption burner. The resulting drops are relatively large which will cause the flame temperature to fluctuate and will scatter the source radiation. This may cause false measurements to be detected. This interference will not happen in the premix burner since fine droplets of sample is produced. d) Flame homogeneity Total consumption burner is used in flame photometry and is not helpful for atomic absorption. This is because the resulting flame is turbulent and non-homogenous, and it combines the function of a nebulizer and burner. Here, oxidant and fuel emerge from separate ports and are mixed above the burner orifices to produce a turbulent flame. Non-homogenous flame is a property that negates its usefulness in atomic absorption, since the flame must be homogeneous, for the same reason that different sample cuvettes in molecular spectrophotometry must be closely matched. One would not want the absorption properties to change from one moment to the next because of the lack of homogeneity in the flame. In a premix burner, the fuel and oxidant are thoroughly mixed inside the burner housing before they leave the burner ports and enter the flame’s primary combustion or inner zone. This type of burner usually produces an approximately laminar (streamlined) flame and is commonly combined with a separate unit for nebulizing the sample. e) Noise Combustion with the premix burners is very quiet, while with Flame selection Disadvantages of flame atomization Only 5 – 15 % of the nebulized sample reaches the flame A minimum sample volume of 0.5 – 1.0 mL is needed to give a reliable reading Samples which are viscous require dilution with a solvent 31 Eletrothermal atomization Graphite furnace technique 32 A basic graphite furnace atomizer is comprised of the following components: graphite tube electrical contacts enclosed water-cooled housing inert purge gas controls A graphite tube is normally the heating element of the graphite furnace. The cylindrical tube is aligned horizontally in the optical path of the spectrometer and serves as the spectrometer sampling cell. A few microliters (usually 5-50) of sample are measured and dispensed through a hole in the center of the tube wall onto the inner tube wall or a graphite platform. The tube is placed between two graphite contact cylinders, which provide electrical connection. An electrical potential applied to the contacts causes current to flow through the tube, the effect of which is heating of the tube and the sample. The entire assembly is mounted within an enclosed, water-cooled housing. Quartz windows at each end of the housing allow light to pass through the tube. The heated graphite is protected from air oxidation by the end windows and two streams of argon. An external gas flow surrounds the outside of the tube, and a separately controllable internal gas flow purges the inside of the tube. The system should regulate the internal gas flow so that the internal flow is reduced or, preferably, completely interrupted during atomization. This helps to maximize sample residence time in the tube and increase the measurement signal. The longitudinally-heated furnace has a major liability. The electrical contacts at each end of the tube must be cooled. As a result, there must always be a temperature gradient along the tube length, the tube ends adjacent to the electrical contacts being cooler than the central portion. This temperature gradient can cause vaporized atoms and molecules to condense as they diffuse to the cooler tube ends. This may produce interferences, the most common type being the incomplete removal of the analyte or matrix from the tube. Incomplete matrix removal during pyrolysis can increase the magnitude of background absorption during atomization. The incomplete removal of analytes during atomization is more serious. It creates "carryover" or "memory", wherein a portion of the analyte in the current sample remains in the tube and contributes to the analytical signal for the following sample. This produces erroneously high analytical results and poor precision. To minimize carryover, most longitudinally-heated furnace heating programs use one or more cleanout steps after the atomization step. A cleanout step involves the application for several seconds of full internal gas flow and a temperature equal to or greater than that used for atomization to remove residual sample components. While this technique works well for the more easily atomized analytes, it is not always successful with those analytes that require higher atomization temperatures. The use of a high-temperature cleanout step may also reduce tube lifetime. Graphite furnace technique process drying ashing atomization 35 Graphite furnace technique Advantages Small sample sizes ( as low as 0.5 uL) Very little or no sample preparation is needed Sensitivity is enhanced ( 10 -10 –10-13 g , 100- 1000 folds) Direct analysis of solid samples 36 Graphite furnace technique Disadvantages Background absorption effects Analyte may be lost at the ashing stage The sample may not be completely atomized The precision was poor than the flame method (5%-10% vs 1%) The analytical range is relatively narrow (less than two orders of magnitude) 37 Cold vapour technique Hg2+ + Sb2+ = Hg + Sb (IV) 38 THE COLD VAPOR MERCURY TECHNIQUE Principle Since atoms for most AA elements cannot exist in the free, ground state at room temperature, heat must be applied to the sample to break the bonds combining atoms into molecules. The only notable exception to this is mercury. Free mercury atoms can exist at room temperature; therefore, mercury can be measured by atomic absorption without a heated sample cell. In the cold vapor mercury technique, mercury is chemically reduced to the free atomic state by reacting the sample with a strong reducing agent like stannous chloride or sodium borohydride in a closed reaction system. The volatile free mercury is then driven from the reaction flask by bubbling air or argon through the solution. Mercury atoms are carried in the gas stream through tubing connected to an absorption cell, which is placed in the light path of the AA spectrometer. Sometimes the cell is heated slightly to avoid water condensation but otherwise the cell is completely unheated. As the mercury atoms pass into the sampling cell, measured absorbance rises indicating the increasing concentration of mercury atoms in the light path. Some systems allow the mercury vapour to pass from the absorption tube to waste, in which case the absorbance peaks and then falls as the mercury is depleted. The highest absorbance observed during the measurement will be taken as the analytical signal. In other systems, the mercury vapour is rerouted back through the solution and the sample cell in a closed loop. The absorbance will rise until an equilibrium mercury concentration is attained in the system. The absorbance will then level off, and the equilibrium absorbance is used for quantitation. The entire cold vapour mercury process can be automated using flow injection techniques. Samples can be analyzed in duplicate at the rate of about 1 sample per minute with no operator intervention. Detection limits are comparable to those obtained using manual batch processes. Advantages of the Cold Vapor Technique The sensitivity of the cold vapour technique is far greater than that of conventional flame AA. First, this improved sensitivity is achieved through a 100% sampling efficiency. All of the mercury in the sample solution placed in the reaction flask is chemically atomized and transported to the sample cell for measurement. The sensitivity can be further increased by using very large sample volumes. Since all of the mercury contained in the sample is released for measurement, increasing the sample volume means that more mercury atoms can be transported to the sample cell and measured. The detection limit for mercury by this cold vapour technique is approximately 0.02 g/L. Although flow injection techniques use much smaller sample sizes, they provide similar performance capabilities, as the entire mercury signal generated is condensed into a much smaller time period relative to manual batch-type procedures. Where the need exists to measure even lower mercury concentrations, some systems offer an amalgamation option. Mercury vapour liberated from one or more sample aliquots is trapped on a gold or gold alloy gauze in the reduction step. The gauze is then heated to drive off the trapped mercury, and the vapour is directed into the sample cell. The only theoretical limit to this technique would be imposed by background or mercury contamination levels in the reagents or system hardware. Limitations to the Cold Vapor Technique Of all the options available, the cold vapour system is still the most sensitive and reliable technique for determining very low concentrations of mercury by atomic absorption. However, the concept is limited to mercury since no other element offers the possibility of chemical reduction to a volatile-free atomic state at room temperature. Hydride generation methods For arsenic (As), antimony (Sb) and selenium (Se) NaBH hea As (V) AsH3 As0(gas) + H2 4 t [H+] in (sol) flame 43 44 HYDRIDE GENERATION TECHNIQUE Principle Hydride generation sampling systems for atomic absorption bear some resemblances to cold vapour mercury systems. Samples are reacted in an external system with a reducing agent, usually sodium borohydride. Gaseous reaction products are then carried to a sampling cell in the light path of the AA spectrometer. Unlike the mercury technique, the gaseous reaction products are not free analyte atoms but volatile hydrides. These molecular species are not capable of causing atomic absorption. To dissociate the hydride gas into free atoms, the sample cell must be heated. In some hydride systems, the absorption cell is mounted over the burner head of the AA spectrometer, and the cell is heated by an air-acetylene flame. In other systems, the cell is heated electrically. In either case, the hydride gas is dissociated in the heated cell into free atoms, and the atomic absorption rises and falls as the atoms are created and then escape from the absorption cell. The maximum absorption reading or peak height or the integrated peak area is taken as the analytical signal. Advantages of the Hydride Technique The elements determinable by hydride generation are listed below. For these elements, detection limits well below the g/L range are achievable. Like cold vapour mercury, the extremely low detection limits result from a much higher sampling efficiency. In addition, separating the analyte element from the sample matrix by hydride generation is commonly used to eliminate matrix-related interferences. Hydride Generation Elements As Bi Ge Pb Sb Se Sn Te The equipment for hydride generation can vary from simple to sophisticated. Less expensive systems use manual operation and a flame-heated cell. The most advanced systems combine the sample chemistries and hydride separation automation using flow injection techniques with a hydride decomposition in an electrically heated, temperature-controlled quartz cell. Disadvantages of the Hydride Technique The major limitation of the hydride generation technique is that it is restricted primarily to the elements listed above. Results depend heavily on various parameters, including the valence state of the analyte, reaction time, gas pressures, acid concentration, and cell temperature. Therefore, the hydride generation technique's success will vary with the operator's care in attending to the required detail. The formation of the analyte hydrides is also suppressed by a number of common matrix components, leaving the technique subject to chemical interference. Monochromator --- diffraction grating 47 Atomic absorption spectrophotometer 48  Interference effects Interferences in atomic absorption can be divided into two general categories, spectral and non-spectral. Non-spectral interferences are those which affect the formation of analyte atoms. NONSPECTRAL INTERFERENCES The first place in the flame atomization process subject to interference is the very first step, the nebulization. If the sample is more viscous or has considerably different surface tension characteristics than the standard, the sample uptake rate or nebulization efficiency may be different between sample and standard. If samples and standards are not introduced into the process at the same rate, it is obvious that the number of atoms in the light beam and, therefore, the absorbance, will not correlate between the two. Thus, a matrix interference will exist. An example of this type of interference is the effect of acid concentration on absorbance. From Figure 3-2, it can be seen that as phosphoric acid concentration increases (and the sample viscosity increases), the sample introduction rate and the sample absorbance decrease. Increased acid or dissolved solid concentrations normally lead to a negative error if it is not recognized and corrected. Matrix interferences can also cause positive errors. The presence of an organic solvent in a sample will produce an enhanced nebulization efficiency, resulting in an increased absorption. One way of compensating for this type of interference is to match the major matrix components of the standard as closely as possible to those of the sample. Any acid or other reagent added to the sample during preparation should also be added to the standards and blank in similar concentrations. Method of Standard Additions There is a useful technique which may make it possible to work in the presence of a matrix interference without eliminating the interference itself, and still make an accurate determination of analyte concentration. The technique is called the method of standard additions. Accurate determinations are made without eliminating interferences by calibrating the concentration in the presence of the matrix interference. Aliquots of a standard are added to portions of the sample, thereby allowing any interferent present in the sample also to affect the standard similarly. The standard additions technique is illustrated in Figure 3-3. The solid line passing through the origin represents a typical calibration line for a set of aqueous standards. Zero absorbance is defined with a water blank, and, as the concentration of analyte increases, a linear increase in absorbance is observed. Let us now take equal aliquots of the sample. Nothing is added to the first aliquot; a measured amount of standard is added to the second; and a larger measured amount is added to the third. The first volume of added standard is usually selected to approximate the analyte concentration in the sample, and the second volume is normally twice the first volume. However, for the method of standard additions to be used accurately, the absorbances for all of the solutions must fall within the linear portion of the working curve. Finally, all portions are diluted to the same volume so that the final concentrations of the original sample constituents are the same in each case. Only  Properly used, the method of standard additions is a valuable tool in atomic absorption. The presence of interference can be confirmed by observing the spiked sample calibration's slope and determining whether it is parallel to the aqueous standard line. If it is not, an interference is present. If an interference is present, the method of standard additions may allow an accurate determination of the unknown concentration by using the standard additions slope for the calibration. However, caution should be used with the technique as it can fail to give correct answers with other types of interference. The method of standard additions will not compensate for background absorption or other types of spectral interference, and normally will not compensate for chemical or ionization types of interference. Chemical Interference A second place where interference can enter into the flame process is in the atomization process. In this step, sufficient energy must be available to dissociate the molecular form of the analyte to create free atoms. If the sample contains a component which forms a thermally stable compound with the analyte that is not completely decomposed by the energy available in the flame, a chemical interference will exist. The effect of phosphate on calcium, illustrated in Figure 3-4, is an example of a chemical interference. Calcium phosphate does not totally dissociate in an air-acetylene flame. Therefore, as phosphate concentration is increased, the absorbance due to calcium atoms decreases. There are two means of dealing with this problem. One is to eliminate the interference by adding an excess of another element or compound which will also form a thermally stable compound with the interferent. In the case of calcium, lanthanum is added to tie up the phosphate and allow the calcium to be atomized, making the calcium absorbance independent of the amount of phosphate. There is a second approach to solving the chemical interference problem. Since the problem arises because of insufficient energy to decompose a thermally stable analyte compound, the problem can be eliminated by increasing the amount of energy; that is, by using a hotter flame. The nitrous oxide-acetylene flame is considerably hotter than air-acetylene and can often be used to minimise chemical interferences for elements generally determined with air-acetylene. The phosphate interference on calcium, for instance, is not observed with a nitrous oxide-acetylene flame, eliminating the need for the addition of lanthanum. Ionization Interference There is a third major interference, however, which is often encountered in hot flames. Figure 3-1 illustrates that the dissociation process does not necessarily stop at the ground-state atom. If additional energy is applied, the ground state atom can be thermally raised to the excited state or an electron may be totally removed from the atom, creating an ion. As these electronic rearrangements deplete the number of ground state atoms available for light absorption, atomic absorption at the resonance wavelength is reduced. When an excess of energy reduces the population of ground-state atoms, an ionization interference exists. Ionization interferences are most common with the hotter nitrous oxide-acetylene flame. In an air-acetylene flame, ionization interferences are normally encountered only with the more easily ionized elements, notably the alkali metals and alkaline earths.  Ionization interference can be eliminated by adding an excess of an element which is very easily ionized, creating a large number of free electrons in the flame and suppressing the ionization of the analyte. Potassium, rubidium, and cesium salts are commonly used as ionization suppressants. Figure 3-5 shows ionization suppression for determining barium in a nitrous oxide-acetylene flame. The increase in absorption at the barium resonance line, and the corresponding decrease in absorption at the barium ion line as a function of added potassium, illustrate the enhancement of the ground state species as the ion form is suppressed. Adding 1000 mg/L to 5000 mg/L potassium to all blanks, standards and samples, the effects of ionization can usually be eliminated. SPECTRAL INTERFERENCES Spectral interferences are those in which the measured light absorption is erroneously high due to absorption by a species other than the analyte element. The most common type of spectral interference in atomic absorption is ‘‘background absorption.’’ Background Absorption Background absorption arises from the fact that not all of the matrix materials in a sample are necessarily 100% atomized. Since atoms have extremely narrow absorption lines, there are few problems involving interferences where one element absorbs at the wavelength of another. Even when an absorbing wavelength of another element falls within the spectral bandwidth used, no absorption can occur unless the light source produces light at that wavelength, i.e., that element is also present in the light source. However, undissociated molecular forms of matrix materials may have broadband absorption spectra, and tiny solid particles in the flame may scatter light over a wide wavelength region. When this type of nonspecific absorption overlaps the atomic absorption wavelength of the analyte, background absorption occurs. To compensate for this problem, the background absorption must be measured and subtracted from the total measured absorption to determine the true atomic absorption component. While now virtually obsolete, an early method of manual background correction illustrates clearly the nature of the problem. With the ‘‘two line method’’, background absorption, which usually varies gradually with wavelength, was independently measured by using a non-absorbing emission line very close to the atomic line for the analyte element, but far enough away so that atomic absorption was not observed, as illustrated in Figure 3-6. By subtracting the absorbance measured at the non-absorbing line from the absorbance at the atomic line, the net atomic absorption was calculated. Nearby, non-absorbing lines are not always readily available; however, inaccuracies in background correction will result if the wavelength for background measurement is not extremely close to the resonance line. Therefore, for accuracy, as well as convenience, a different method was needed. Continuum Source Background Correction Continuum source background correction is a technique for automatically measuring and compensating for any background component which might be present in an atomic absorption measurement. This method incorporates a continuum light source in a modified optical system, illustrated in Figure 3-7. The broadband continuum (‘‘white’’ light) source differs from the primary (atomic line) source in that it emits light over a broad spectrum of wavelengths instead of at specific lines. Figure 3-8 shows that atomic absorption, which occurs only at very discrete wavelengths, will not measurably attenuate the emission from the continuum source. However, background absorption with very broad absorption spectra will absorb the continuum emission and the line emission. As shown in Figure 3-7, light from both the primary and continuum lamps is The two lamps are observed by the detector alternately in time, and as illustrated in Figure 3-9, instrument electronics separate the signals and compare the absorbance from both sources. An absorbance will be displayed only where the absorbance of the two lamps differs. Since background absorption absorbs both sources equally, it is ignored. True atomic absorption, which absorbs the primary source emission and negligibly absorbs the broad band continuum source emission, is still measured and displayed as usual. Figure 3-10 shows how background absorption can be automatically eliminated from the measured signal using continuum source background correction. In the example, a lead determination is shown without background correction (A) and with background correction (B). Both determinations were performed at the Pb 283.3 nm wavelength with 15x Limitations of Continuum Source Background Correction 1. Requires additional continuum light source(s) and electronics. 2. Requires the intensities of the primary and continuum sources to be similar. 3. Two continuum sources are required to cover the full wavelength range. 4. Requires critical alignment of the continuum and primary sources for accurate correction. 5. May be inaccurate for structured background absorption. Zeeman Background Correction For those applications where the limitations of the continuum source approach are significant to the analysis, the Zeeman background correction system may be preferable. Zeeman background correction uses the principle that the electronic energy levels of an atom placed in a strong magnetic field are changed, thereby changing the atomic spectra that measure these energy levels. When an atom is placed in a magnetic field and its atomic absorption profile observed with polarized light, the normal single-line atomic absorption profile is split into two or more components symmetrically displaced about the normal position, as illustrated in Figure 3-11. The spectral nature of background absorption, on the other hand, is usually unaffected by a magnetic field. By placing the poles of an electromagnet around the atomizer and making alternating absorption measurements with the magnet off and then on, the uncorrected total absorbance (magnet off) and ‘‘background only’’ absorbance (magnet on) can be made, as in Figure 3-12. The automatic comparison made by the instrument to compensate for background correction is similar to that for the continuum source technique, except that only one atomic line source is used. As a result, there are no potential problems with matching source intensities or coincident alignment of optical paths. Also, background correction is made at the analyte wavelength rather than across the entire spectral bandwidth, as occurs with continuum source background correction. With Zeeman background correction, the emission profile of the line source is identical for both AA and background measurements. As a result, most complex structured background situations can be accurately corrected with Zeeman background correction. This can be seen in Figure 3-13, where background absorption due to the The examples used above to illustrate Zeeman effect background correction are based on the use of a transverse AC Zeeman system, the type most commonly used with commercial AA instrumentation. However, there are three types of Zeeman effect background correction systems available on commercial atomic absorption instruments: DC Zeeman, transverse AC Zeeman and longitudinal AC Zeeman. These systems differ in how the magnetic field is applied and by the means used to measure the combined (atomic absorption plus background absorption) and background absorption-only signals. DC Zeeman systems use a permanent magnet and a rotating or vibrating polarizer to separate the combined and background- only signals. AC systems use an electromagnet, and measure the combined and background-only signals by turning the magnetic field on and off. The difference between transverse (magnetic field applied across the light path) and longitudinal (magnetic field applied along the light path) AC Zeeman Advantages of Zeeman Effect Background Correction 1. Corrects for high levels of background absorption. 2. Provides accurate correction in the presence of a structured background. 3. Provides true double-beam operation. 4. Requires only a single, standard light source. 5. Does not require intensity matching or coincident alignment DC Zeeman of multiple Systemssources. Advantages: Less expensive to operate (lower power consumption) Disadvantages: Has poorer sensitivity and analytical working range relative to AC Zeeman systems. The polarizer reduces light throughput by as much as 50%, affecting analytical performance. A mechanical assembly is required to rotate or vibrate AC Zeeman Systems Advantages: Offers better sensitivity and expanded analytical working ranges relative to DC Zeeman systems. No polarizer is required, so it provides higher light throughput and improved analytical performance. (Longitudinal AC Zeeman systems only) Requires no additional mechanical devices. Disadvantages: Requires more electrical power than DC Zeeman systems, so has higher operating expenses. The polarizer causes reduced light throughput by as much as 50%, affecting analytical performance. (Transverse AC Zeeman systems only.) Physical interference Flame Spray efficiency fluctuations due to difference in viscosity and surface tension between the standard and sample. Furnace Sample dispersion ; Measurement value fluctuations due to tube temperature distribution Viscosity within the graphite furnace ; Adherence to sample tip causing errors in collection quantity. Example: samples, such as blood or juice, containing numerous organic components.  Spectral interference Spectral absorption line overlapping with the absorption line of the target element. Absorption and scattering by molecules  Spectral interference ectral absorption line overlapping with the absorptio e of the target element. Target element Spectral line Interfering Spectral line (nm) element (nm) Al  V  Ca  Ge  Cd  As  Co  In  Cu  Eu  Fe  Pt  Ga  Mn  Hg  Co  Mn  Ga  Sb  Pb  Si  V  Zn  Fe  Chemical interference Generation of non-separable compounds by coexisting matrices Example : influence of PO4-, SO4-, SiO2 relative to Ca, Mg in flame analysis (generation of Ca2PO4) Generation of low boiling point compounds by coexisting matrices Example: influence of chloride ions relative to Cd in furnace analyses (generation of CdCl2)  Matrix modifier effect Masking of obstructing matrices Influence of phosphate on Ca is masked by La Conversion of obstructing matrices to compounds that easily undergo sublimation or evaporation Sublimation agent Example: removal of chloride ion by ammonium salt of nitric acid or phosphoric acid Conversion of measured elements to stable oxides or metallic intermediary compounds Stabilizing agent: Example: creation of measured element alloy using white metals (Pd, Pt, Rh) Application examples of the matrix modifier method Atomic emission spectroscopy Atomic emission spectroscopy Historically, many techniques based on emission have been used Flame and electrothermal methods now widely superseded by Inductively- Coupled Plasma (ICP) method Developed in the 1970s Higher energy sources than flame or electrothermal methods 77  ICP-AES/OES Inductively coupled plasma-atomic emission spectroscopy (or optical emission spectroscopy) Offer several advantages over flame/electrothermal: Lower inter-element interference (higher temperatures) With a single set of conditions signals for dozens of elements can be recorded simultaneously Lower LOD for elements resistant to decomposition Permit determination of non-metals (Cl, Br, I, S) Can analyse concentration ranges over several decades (vs 1 or 2 decades for other methods) Disadvantages: More complicated and expensive to run Require higher degree of operator skill 78 Modern ICP-OES spectrometer Over 70 elements (in principle simultaneously) Including non-metals such as sulfur, phosphorus, and halogens (not possible with AAS) ppm to ppb range Principle: Argon plasma generates excited atoms and ions; these emit characteristic radiation 79 ICP-AES Instrumentation 80 Components for sample injection and the ICP torch Up to 7000°C www.cleanwatertesting.com/news_N www.midwestrefineries.com/re R149.htm finingandassaying.htm 81 Meinhard nebuliser Caution: The capillary is easy to block and difficult to unblock 82 ICP torch water cooled induction coil powered by RF generator (2 kW power at 27 MHz) concentric quartz tubes 11-17 L/min d=2.5 cm 83  Torch Ignition Sequence Ionisation of Argon initiated by spark from Tesla coil Start gas flow Switch on RF power After leaving injector, sample moves at high velocity Punches hole in centre of plasma Plasma generated 84  Atomisation / Ionisation In plasma, sample moves through several zones Preheating zone (PHZ): temp = 8000 K: Desolvation/evaporation Initial radiation zone (IRZ): 6500-7500 K: Vaporisation, Atomisation Normal analytical zone (NAZ): 6000-6500 K: Ionisation 85  ICP INTERFERENCES ► Spectral interferences:  caused by background emission from continuous or recombination phenomena,  stray light from the line emission of high concentration elements,  overlap of a spectral line from another element,  or unresolved overlap of molecular band spectra. ► Corrections  Background emission and stray light compensated for by subtracting background emission determined by measurements adjacent to the analyte wavelength peak.  Correction factors can be applied if interference is well characterized  Inter-element corrections will vary for the same emission line among instruments because of differences in resolution, as determined by the grating, the entrance and exit slit widths, and by the order of dispersion.  Physical interferences of ICP ► cause  effects associated with the sample nebulization and transport processes.  Changes in viscosity and surface tension can cause significant inaccuracies, ► especially in samples containing high dissolved solids ► or high acid concentrations.  Salt buildup at the tip of the nebulizer, affecting aerosol flow rate and nebulization. ► Reduction  by diluting the sample or by using a peristaltic pump,  by using an internal standard or by using a high solids nebulizer.  Interferences of ICP ► Chemical interferences:  include molecular compound formation, ionization effects, and solute vaporization effects.  Normally, these effects are not significant with the ICP technique.  Chemical interferences are highly dependent on matrix type and the specific analyte element.  Memory interferences: ► When analytes in a previous sample contribute to the signals measured in a new sample. ► Memory effects can result  from sample deposition on the uptake tubing to the nebulizer  from the build up of sample material in the plasma torch and spray chamber. ► The site where these effects occur is dependent on the element and can be minimized  by flushing the system with a rinse blank between samples. ► High salt concentrations can cause analyte signal suppressions and confuse interference tests.  Advantages of plasma Prior to observation, atoms spend ~ 2 sec at 4000-8000 K (about 2-3 times that of hottest combustion flame) Atomisation and ionisation is more complete Fewer chemical interferences Chemically inert environment for atomisation Prevents side-product (e.g. oxide) formation Temperature cross-section is uniform (no cool spots) Prevents self-absorption Get linear calibration curves over several orders of magnitude 90  Applications ICP-OES used for quantitative analysis of: Soil, sediment, rocks, minerals, air Geochemistry Mineralogy Agriculture Forestry Forensics Environmental sciences Food industry Elements not accessible using AAS Sulfur, Boron, Phosphorus, Titanium, and Zirconium 91